now that we're in the correct section, i think its a good time to pimp this link:
http://www.240sxtech.com
this is where the articles written by our members regarding all sorts of topics are collected. check it out next time you run into a problem!
is just your HUD unit burnt out or is it the digital speedo too?
if its the speedo, check this article for help.
if not, try and find a new HUD unit with the wire/plug that goes into the back of your cluster. those are all the same regardless of year. you don't need to swap the entire cluster, but if you decide to, you can keep your tachometer by swapping it to the new cluster.
I recommend testing any cluster or speedo/HUD display you might buy before giving up your money because a lot of them are in no better condition than yours.
good luck!